1) Field of the Invention
This invention relates to a three-phase hybrid AC-voltage/AC-voltage converter (xe2x80x9chybrid direct converterxe2x80x9d) for the generation of an inductive load-storing, pulse-width modulated three-phase voltage of freely selectable amplitude and frequency of the fundamental frequency, whose implementation requires a minimum number of interruptible power semiconductors, as described in the introductory clause of a first aspect.
2) Description of the Related Art
In accordance with the conventionally arts, for direct pulse-width controlled energy conversion, i.e. one having no intermediate circuit storage elements, between three-phase systems, or, with the advantage of having a lower number of interruptible power semiconductors, a combination of an output-side three-phase intermediate voltage circuit and an input-side pulse converter located on the three-phase intermediate circuit is used, which enables complete switching off of the individual bridge arms. Such a converter configuration, which is preferred because of its lower implementation cost over the matrix type converter, is described in the Austrian patent application xe2x80x9cDevice for a quasi-direct pulse-width controlled energy conversion between three-phase systemsxe2x80x9d, application date Jul. 27, 2001 (inventors: Kolar/Ertl). For the implementation of a bridge arm of the input-side power converter, 3 interruptible power semiconductors are used, thereby the AC-voltage/AC-voltage converter has a total of 15 irreversible power semiconductors (IGBTs or MOSFETs). The control of the system that, as a result of the missing intermediate circuit, is to be considered as a direct converter is effected so that an interlinked line voltage is connected to the input of the output-side intermediate circuit voltage converter, which then, by an appropriate pulse-width modulation, is converted into the desired three-phase voltage system that supplies an inductive, i.e. current-injecting, consumer. As a result, segments of the continuous consumer-phase currents arise at the input of the intermediate voltage circuit pulse converter and are routed via the input-side pulse converter into the network. By an appropriate changeover, a sinusoidal power input can be obtained between the interlinked network-phase voltages after suppressing frequent spectral portions by an input filter. As the output-side pulse converter commutates on the DC side and the input-side pulse converter on the power network, i.e. AC side, the converter may be designated as a three-phase hybrid AC-voltage/AC-voltage converter.
The changeover of the input-side power converter must occur so that there will be no short-circuiting of two network phases, that is, it must not be done with overlapping. In this type of changeover, though, the last load-inductivity impressing input current of the intermediate voltage circuit converter is interrupted, for which reason, to limit the cut-off voltage of the power semiconductors, a surge voltage protector has to be provided at the input of the intermediate voltage circuit that carries the current in the changeover interval.
As previously mentioned, the hybrid direct converter has a lower complexity compared to a matrix type converter, but requires with its 15 power transistors 3 power transistors more than a direct voltage-side coupling of two intermediate voltage circuit converters with intermediate circuit capacitor, each requiring 6 power transformers (and anti-parallel diodes), which in principle will avoid the problem off switching surges.
It is an object of the present invention to provide a hybrid direct converter with only 12 interruptible power semiconductors (in combination with power diodes) and a process for controlling this device, which in principle avoids the occurrence of principle-caused switching surges, or which is a high commutation safety.
The basic idea of the invention, for implementation of the interruptible bridge arms of the input section of the hybrid direct converter, is to use two symmetrical interruptible electronic switches each, whose implementation only requires one power transistor each. The output-side intermediate voltage circuit converter of the hybrid direct converter is retained unchanged, as a result of which the overall configuration will only have 12 interruptible valves.
The device constructed in this way can in principle be controlled in the same way as the conventional hybrid direct converter. If, however, according to a second aspect, before switching off a bridge arm of the input-side converter and switching of another bridge arm, i.e. a change of the interlinked system voltage applied at the input of the output-side intermediate voltage circuit converter, the output-side intermediate voltage circuit converter is switched into the free-wheeling state, i.e. all power transistors of the output-side intermediate voltage circuit converter connected to the positive input voltage rail are made conductive (and all power transistors connected to the negative input voltage rail are blocked), or all power transistors of the output-side intermediate voltage circuit converter connected to the negative input voltage rail are connected through (and all power transistors connected to the negative input voltage rail are blocked), the input current of the output-side intermediate voltage circuit converter becomes zero and the conversion of the bridge arms of the input-side system part according to the invention can occur at zero-current, whereby a switching surge is safely avoided and no overvoltage protection has to be provided. After switching over the input-side system part, the free-wheeling state of the output-side intermediate voltage circuit converter is terminated and the circuit state required for generating the desired output voltage is assumed. This conversion strategy may, in its unchanged form, also be used for the well-known hybrid direct converter.
These and other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.